BOREAL FOREST PLANTS TAKE UP ORGANIC NITROGEN

Plant growth in the boreal forest, the largest terrestrial biome, is g enerally limited by the availability of nitrogen. The presumed cause o f this limitation is slow mineralization of soil organic nitrogen(1,2) . Here we demonstrate, to our knowledge for the first time, the uptake of organic nitrogen in the field by the trees Pinus sylvestris and Pi cea abies, the dwarf shrub Vaccinium myrtillus and the grass Deschamps ia flexuosa. These results show that these plants, irrespective of the ir different types of root-fungal associations (mycorrhiza), bypass ni trogen mineralization. A trace of the amino acid glycine, labelled wit h the stable isotopes C-13 and N-15, was injected into the organic (mo r) layer of an old successional boreal coniferous forest. Ratios of C- 13:N-15 in the roots showed that at least 91, 64 and 42% of the nitrog en from the absorbed glycine was taken up in intact glycine by the dwa rf shrub, the grass and the trees, respectively. Rates of glycine upta ke were similar to those of N-15-ammonium. Our data indicate that orga nic nitrogen is important for these different plants, even when they a re competing with each other and with non-symbiotic microorganisms. Th is has major implications for our understanding of the effects of nitr ogen deposition, global warming and intensified forestry.